1. Field of the Invention
The present invention relates to a mold clamping system for use in an injection molding machine, a die casting machine or the like, and more particularly to the construction of a straight-hydraulic mold clamping system capable of keeping the platens in parallel.
2. Description of the Prior Art
A conventional straight-hydraulic mold clamping system for use in an injection molding machine etc. comprises a bed, a cylinder for mold clamp disposed horizontally on the bed, and a stationary platen and a movable platen disposed in the reciprocating direction of the piston in the mold clamp cylinder respectively and attached to tie bars. The mold clamp cylinder has a piston reciprocating within the inner chamber of the mold clamp cylinder, a rod projecting to the outside of the mold clamp cylinder, and a bearing portion for the rod. The movable platen is fastened to the rod. The rod bearing portion is constituted by a bushing having a clearance of at least 30.mu. for allowing advance or retreat of the rod. Therefore, there is a fair possibility of the rod being eccentric within the range of the clearance of the bearing portion and simultaneously of the tie bars being bent down by the weights of the movable platen and a molding die attached thereto, with the result that parallelism of the movable and stationary platens deteriorates. If this parallelism deteriorates, an offset load is exerted on guide pins etc. of the molding die and gives rise to a wear phenomenon, registration of the molding dies cannot completely be effected and consequently, the service life of the molding dies is shortened.
In order to eliminate the aforementioned drawbacks, there has heretofore been adopted either a method of making the diameter of the tie bars large so as to minimize the deflection of the tie bars or a method of interposing a slide plate between the movable platen and the bed.
In the former method, however, is a limit to the increase of the diameter of the tie bars because the larger the diameter of the tie bars, the smaller the area of the platen surface to which a molding die is to be attached. Accordingly, if the surface area of the platen is enlarged the full load exerted on the system is undesirably increased. Therefore, the diameter of the tie bars can only be made larger within the range which will not occupy the surface area of the platen to which the molding die is to be attached.
Further, in the latter method, the parallelism of the mold clamp cylinder, stationary platen and movable platen provided with the slide plate must be retained. However, since the bed of the system is not necessarily horizontal and since the bearing portion has a clearance, it is difficult to precisely obtain the aforementioned parallelism from a standpoint of mechanical technology. Assuming that the parallelism can precisely be obtained, there are disadvantages that the slide plate is thermally affected during a molding cycle and will change in dimension, that since there is formed between the slide plate and the bed a clearance necessary for the sliding of the slide plate and since the slide plate will be worn due to metallic contact with the bed, the clearance must be adjusted in accordance with the frequency of use while allowing for errors, and that the slide plate is required to be slightly adjusted in accordance with the weight of a molding die every time the molding die is changed. In any event, the limit of the deviation from parallelism of the platens is 50.mu. at most and cannot be guaranteed over a long period of time. In the present state, therefore, spotting of the molding dies is carried out in consideration of the aforementioned deviation from parallelism of the platens.
In proportion as engineering plastics is developed, there is now an increased demand for accuracy on the order of microns in injection molding. In order to fulfill this demand, molding dies are absolutely required to cause their parting surfaces to precisely register with each other. Even though the molding dies are precisely finished, if there are errors in mold registration, it will be impossible to effect molding with accuracy on the order of microns. In this sense, since the limit of the deviation from parallelism of the platens of the conventional mold clamping system is 50.mu. at most, the molding dies of the system cannot be used at all for precision parts which are required not to have accuracy deviation of more than 10.mu..